1. Field of the Invention
The present invention relates to a method and apparatus for adjusting a frequency characteristic of a signal, and in particular to the method and system that can be reduced into practice, for example, as an equalizer to adjust the frequency characteristic of an audio signal.
2. Description of Related Art
In general, an audio system with which a user can enjoy music and movies recorded on compact discs (CDs) and DVDs (Digital Versatile Discs) is provided with a graphic equalizer. The graphic equalizer includes circuitry to divide an audio signal into signal components falling into a plurality of frequency bands and to enhance (boost) or attenuate (cut) the level of each signal component in each frequency band. Such adjustment for each frequency band enables the frequency characteristic of an audio signal to be adjusted, so that the frequency characteristic agrees with the acoustic characteristic of speakers and other audio devices, and is matched with environments in which music or movies are appreciated.
A multi-channel surround-sound audio system, which has been in widespread use recently, is composed of five to eight channels, for example and provided with an amplifying unit to control audio outputs from those channels and speakers in charge of audio output from each channel.
The multi-channel surround-sound audio system realizes a surround-sound reproduction by adequately positioning the speakers. In this system, it is required that a sound pressure level and a frequency characteristic of each audio signal are appropriately set to match the characteristic of each speaker.
In order to meet such requirement, there are provided an equalizer and an amplifier, channel by channel, in most of the amplifying units of the multi-channel surround-sound audio systems.
On the other hand, the multi-channel surround-sound audio systems include an audio system provided with an automatic sound-field correction system to automatically correct a sound pressure level and a frequency characteristic of each audio signal.
The automatic sound-field correction system is able to detect and analyze the positions and characteristics of speakers and environments of a room in which the speakers are placed and to use the analyzed results so that a frequency characteristic (i.e., a target frequency characteristic) to be given to an audio signal in each channel is calculated. And the correction system is capable of enhancing or attenuating a level of a signal component in each band of each audio signal, thus atomically controlling the frequency characteristic of each audio signal.
However, the conventional equalizer has suffered the situation where the larger the number of bands (i.e., the number of divided frequency bands), the higher resolution in frequency, thus more accurate adjustment of the frequency characteristics, while the higher manufacturing cost for audio systems.
In contrast, in the conventional equalizer, if the number of bands is lessened, the manufacturing cost will be reduced, but the frequency resolution will become deteriorated, thus lowering accuracy in the frequency characteristic adjustment. That is, there is a confliction between sustaining or improving accuracy in the frequency characteristic adjustment and reducing the manufacturing cost of audio systems.
Further, the conventional audio system with the equalizer is also provided with amplifiers and attenuators placed after the equalizer in order to increase or lower, as a whole, the levels of audio signals outputted from the equalizer for adjustment of changes in the audio signals occurring due to control in the equalizer. In this case, however, when the levels of the audio signals from the equalizer are raised by the amplifiers, there occurs a problem that noise mixed with the audio signals is also amplified together. This lowers a signal to noise ratio in an audio system, thus deteriorating sound reproduction with higher quality.